After all, was there ever a Lieutenant Caveman to obtain promotion to the rank of Captain? Has Captain Marvel any hope of obtaining the rank of Major Marvel or Lieutenant Colonel Marvel? Was Captain Planet once an ensign? And might he ever be known as Rear Admiral Planet?

The resolution to this quandary lies in the original meaning of 'Captain,' that of the highest ranking of men still in the field. Admirals and Generals are easily imagined as too old and salty to take an enemy hand-to-hand, instead governing battles from behind; whilst lesser ranks speak of lesser authority, and perhaps the green recruit not yet proven in skill and savvy.

But let us let history come alive for a moment. "Captain" descends to us from ye Olde French capitaine, by turns from later Latin capitāneus, from caput (“head”), which gives to us words such as "capital" and "decapitate." The captain has, historically, been the leader of a unit of men in battle, the title often accorded to the highest ranking man whose place in the business of bloodshed was on the battlefield, instead of behind the lines directing the business at a more strategic level from afar. And, because ships needed someone on them to give commands, even if those ships were to be right at the forefront of battle - and because the early military ships had small crews, not much beyond the size of a field unit in a land campaign anyway, their heads were captains as well. The tradition stuck even as a dichotomy was spun between land and sea forces, with land captains remaining the same in general description while larger and more complex naval vessels still generally fell under a captain, while the expanse of their operations raised the prestige of the captain of a ship.

Modernly, the military ship's captain is a much higher rank, just below a Commodore or some species of Admiral depending on which navy; while the equivalent officer in an army would be a Colonel, falling below the varied forms of General, and above Lieutenant Colonel, then Major, before descending to captaincy. And, it might be pointed out as well, that large ships (such as an aircraft carrier group) will in fact tend to have some lesser sort of admiral on board topping the ladder of authority.

Now, Captain Hook, Captain Kidd, and Captain Kirk (and his further past and future counterparts Captain Archer, Captain Pike, Captain Picard, and Captain Janeway) all came by their captaincies the more-or-less honest way, promotion up through lesser ranks. Kirk, at least, spent the bulk of the reimagined origin story making up that most recent Star Trek film installment as a Lieutenant, before a highly unlikely jump-up in rank at the end (though the destruction of much of Starfleet in that battle over Vulcan must've left them smarting for captains). And, at later points in his career (as shown in the original film series) he was indeed promoted to Admiral -- then demoted to Captain and eventually back up to Admiral again. Such piratelords as Kidd and Hook, well they may not have risen through the same rank structure, they surely commanded ships. A plot point in Pirates of the Caribbean arose where Jack Sparrow, long deprived by mutiny of his command of the Black Pearl, complained that he had not received the bounty for which he'd sold his soul to Davy Jones -- thirteen years of captaincy; Jones reminded Sparrow, for all his years in effective exile from this office, he had still introduced himself as "Captain Jack Sparrow."

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Many thanks to The Custodian's 'somewhat porous memory' for providing the following (to his credit he did suggest I find actual references):

"Note that 'Captain Kirk of the Enterprise' isn't technically a rank, it's a position. In modern use, the CO of a ship is called 'Captain' but on smaller ships he might be ranked Lieutenant Commander, and on carriers he is likely a 'captain' or (rarely) a VADM or some such. So using naval/space naval titles might be confusing! .... The commanding officer of the vessel is ALWAYS called 'Captain.' To prevent confusion, if other officers aboard as passengers have an actual rank of 'captain' they are 'courtesy promoted' and referred to as the next highest rank. So e.g. a Marine Captain aboard an amphib assault ship is always called 'Major.' Kirk was the Captain of Starfleet's flagship, akin to being a carrier captain. They actually have the rank of Captain, as well as the position. The highest ranking person on the vessel does not get to issue commands. The commanding officer (always referred to as Captain) has both the responsibility and the authority to give orders aboard ship. So if an admiral wants something done aboard a ship, they can ask and it'll almost always get done. If the Captain countermands their order, the sailor will obey the captain UNTIL AND UNLESS the admiral then removes the captain from his position."